1. Field of the Invention
The present invention relates generally to a method and apparatus for configuring and controlling a two-cylinder, reciprocating pump such that it pumps a constant volumetric flow rate at any discharge pressure. Such a pump is useful for applications such as chromatography, which requires accurate, constant flow rates from pumps at high and variable discharge pressure. More particularly the present invention utilizes a constant rotational speed for most of a given cycle, but utilizes pressure control (and slightly varying rotational speed) for a portion of the cycle in which the flow would normally increase if rotational speed is maintained a constant. As a consequence, the pressure control acts to control the rotational speed to a value very close to the previous speed. Another advantage is a largely constant volumetric input flow rate which improves accuracy of low pressure gradient forming. A logic system is incorporated to provide accurate low pressure gradient former switching, corrected for the depressurization of each of the pump cylinders before their refill.
2. Background Art
Pumps used for liquid chromatography have stringent requirements to deliver constant and accurate flow rates over a range of discharge pressures. Reciprocating (or piston) pumps are usually employed for this purpose for reasons including their relatively fixed displacement.
Present-day piston pumps for chromatography are often two-cylinder pumps where the pistons are actuated by cams—usually one cam per piston. The cam profiles are designed so that the sum of the positive (pumping) speeds of the pistons is a constant if the rotational speed is constant. As one piston decelerates near the end of its delivery stroke, the other cylinder has finished filling and the piston accelerates as it begins its stroke toward the cylinder head. The result of the constant sum of piston velocities in this interval is, if both discharge check valves are open, the total flow rate is constant.
When the pump discharge pressure is low (less than about 10 atmospheres, absolute), the flow rate remains constant with constant rotational speed. Liquids are, in fact, not truly incompressible. A measure of a fluid's “compressibility” is the bulk modulus of elasticity, Ev, defined as
      E    v    =            -              V        _              ⁢                  ⅆ        p                    ⅆ                  V          _                    where p is pressure and  is volume. The bulk modulus of elasticity is larger for a liquid than a gas, but is not infinite. According to the above equation, when pressure changes, so does the volume of a liquid. Because of this volume change, and because the components of the pump are non-rigid, the actual travel distance from bottom dead center until the discharge check valve opens becomes non-negligible. The result is, during this portion of the cycle, fluid is being pumped from a single cylinder only, while the shape of the cam profile is increasing or keeping a piston's speed constant. Thus, at a constant rotational speed, the flow rate is less than the constant value required. Early pumps used an accumulator to smooth the pressure and flow rate in time. In present-day pumps, the instantaneous flow deficiency is made up by increasing the rotational speed of the pump by the control system in this region.
To maintain a constant flow rate, the pump discharge pressure should remain constant. Present-day pumps use either a flow measurement or pressure measurement as a process variable within a high-speed control system to maintain constant flow rate. The manipulated variable is the pump's rotational speed. So, during the period in which a piston is traveling toward its head while its discharge check valve is closed, the pump rotational speed must increase.
Control of the speed of the pump is based on pump discharge pressure most of the time, despite the fact that constant speed is required through the majority of the pump's cycle. Control based on pressure is subject to the noise and response time of the sensor. In addition, the flow rate into the pump during the inflow parts of the pump cycle is not well controlled, making it difficult to produce accurate chromatography eluant composition gradients by repetitively and synchronously switching eluant compositions at the pump inlet. This is referenced in the art as “low pressure gradient forming” and has the advantage that it requires only a single high-pressure pump, rather than the usual dual pump.
There is, therefore, a need for a pump to accurately deliver a constant flow rate fluid of controlled (gradient) composition regardless of the discharge pressure.